An anti-anxiety drug makes fish eat faster and become less social, according to a study in Science. The results raise the possibility that traces of this medication in wastewater could change the behavior of wild fish.

Use of benzodiazepines, a type of anxiety-reducing drug, is now common worldwide. These drugs remain in treated wastewater, and some don’t easily degrade after being washed into rivers and streams.

The researchers sampled water in Sweden for a benzodiazepine called oxazepam. Wastewater effluent contained 0.73 micrograms per liter of the drug, while the River Fyris contained 0.58 micrograms per liter. The drug seemed to build up in the tissues of fish, since the muscles of European perch from that river had 3.6 micrograms per kilogram of oxazepam.

The team then placed perch in water containing a low amount of oxazepam. After a week, the fish had oxazepam levels in their muscle tissue similar to those found in the River Fyris fish. When the researchers tested the perch’s personality traits, they found that the fish swam more and spent less time near other fish after being exposed to the drug. The perch also gobbled up zooplankton faster.

“Changes in fish feeding rate may, over time, have ecosystem-level consequences,” the authors warn. Being more active and less social could make fish more likely to be caught by predators. And the fact that one drug can change fish behavior “is alarming, considering the cocktail of different pharmaceutical products that are found in waters worldwide,” the team writes. — Roberta Kwok | 14 February 2013